The invention relates to a photopolymerizable mixture comprising a polymeric binder, a polymerizable compound, in particular an acrylate or alkacrylate, and a photoinitiator combination.
Photopolymerizable mixtures are known in the art which, in order to increase the photosensitivity, contain certain combinations of photoinitiators and activators, for example combinations of carbonyl group-containing initiators and tertiary amines. Such mixtures with a synergistic action are described, for example, in U.S. Pat. Nos. 3,759,807, 4,054,682 and 4,071,424. A disadvantage of these mixtures which contain low-molecular-weight amines is that they have a relatively short shelf life, since the amines can easily bleed out, in particular from thin layers.
In JP-A 50/129,214, registered on 2 Apr. 1974 under the number 49/36,614, a photopolymerizable mixture is described which contains a tetra(meth)acrylate of a N,N,N',N'-tetrahydroxyalkyl-alkylenediamine as a polymerizable compound. The tetrafunctional compound serves as a cross-linking agent.
It is furthermore known that the free-radical polymerization of ethylenically unsaturated compounds can be initiated by irradiation with visible light in the presence of photoreducible dyes and reducing agents, for example amines (U.S. Pat. No. 3,097,096). However, these initiator combinations are essentially employed only in aqueous solution or in combination with water-soluble binders.
Initiator combinations of photoreducible dyes and other reducing agents are described in U.S. Pat. Nos. 3,597,343 and 3,488,269. Photopolymerizable mixtures which contain exclusively photoreducible dyes as initiators have hitherto not been employed in practice due to their unsatisfactory photosensitivity.
In JP-A 54/151,024 a photopolymerizable mixture is described which contains an initiator combination of a merocyanine dye and a trihalomethyl-substituted s-triazine and is sensitive towards visible light, for example an argon laser. However, the sensitivity of these mixtures towards visible laser light is inadequate for economic utilization.
The earlier German Patent Application P 37 10 281.8 describes photopolymerizable mixtures which contain (meth)acrylates having urethane groups, tertiary amino groups and optionally urea groups in their molecules; polymeric binders; and, as photoinitiators, a combination of a photoreducible dye, a radiation-sensitive trihalomethyl compound and an acridine, phenazine or quinoxaline compound.
U.S. Pat. No. 3,717,558 describes metallocenes formed from elements of the Subgroups IIa to VIIIa in combination with a further photoinitiator having a carboxylic acid chloride group, for use in photopolymerizable recording materials. These initiator combinations are very sensitive to oxygen and susceptible to hydrolysis and are thus less suitable for the production of printing plates and photoresist materials. Further metallocenes and their use as photoinitiators in photopolymerizable mixtures are described in EP-A 119 162, EP-A 122 223 and EP-A 186 626. These metallocenes comprise titanocenes which have a good resistance to air and show a spectral sensitivity ranging from UV light up to visible light. However, they are only sparingly soluble or even insoluble in the solvents customarily used in the coating process. In EP-A 242 330 and EP-A 269 573, photopolymerizable mixtures are moreover described, which contain a photoinitiator composition comprising a titanocene and a liquid photoinitiator of a hydroxyl or aminoacetophenone type. Also in these materials, the achievable photosensitivity for rapid imaging by means of a low-energy and, at the same time, low-cost argon ion laser is insufficient.